The RNA helicase DDX39 contributes to the nuclear export of spliceosomal U snRNA by loading of PHAX onto RNA.

RNA helicases are involved in RNA metabolism in an ATP-dependent manner. Although many RNA helicases unwind the RNA structure and/or remove proteins from the RNA, some can load their interacting proteins onto RNAs. Here, we developed an in vitro strategy to identify the ATP-dependent factors involved in spliceosomal uridine-rich small nuclear RNA (U snRNA) export. We identified the RNA helicase UAP56/DDX39B, a component of the mRNA export complex named the transcription-export (TREX) complex, and its closely related RNA helicase URH49/DDX39A as the factors that stimulated RNA binding of PHAX, an adapter protein for U snRNA export. ALYREF, another TREX component, acted as a bridge between PHAX and UAP56/DDX39B. We also showed that UAP56/DDX39B and ALYREF participate in U snRNA export through a mechanism distinct from that of mRNA export. This study describes a novel aspect of the TREX components for U snRNP biogenesis and highlights the loading activity of RNA helicases.

The hnRNP C tetramer binds to CBC on mRNA and impedes PHAX recruitment for the classification of RNA polymerase II transcripts.

In eukaryotic cells, various classes of RNAs are exported to the cytoplasm by class-specific factors. Accumulating evidence has shown that export factors affect the fate of RNA, demonstrating the importance of proper RNA classification upon export. We previously reported that RNA polymerase II transcripts were classified after synthesis depending on their length, and identified heterogeneous nuclear ribonucleoprotein (hnRNP) C as the key classification factor. HnRNP C inhibits the recruitment of PHAX, an adapter protein for spliceosomal U snRNA export, to long transcripts, navigating these RNAs to the mRNA export pathway. However, the mechanisms by which hnRNP C inhibits PHAX recruitment to mRNA remain unknown. We showed that the cap-binding complex, a bridging factor between m7G-capped RNA and PHAX, directly interacted with hnRNP C on mRNA. Additionally, we revealed that the tetramer-forming activity of hnRNP C and its strong RNA-binding activity were crucial for the inhibition of PHAX binding to longer RNAs. These results suggest that mRNA is wrapped around the hnRNP C tetramer without a gap from the cap, thereby impeding the recruitment of PHAX. The results obtained on the mode of length-specific RNA classification by the hnRNP C tetramer will provide mechanistic insights into hnRNP C-mediated RNA biogenesis.

ISG20 and nuclear exosome promote destabilization of nascent transcripts for spliceosomal U snRNAs and U1 variants.

Primary RNA transcripts are processed in a plethora of ways to become mature functional forms. In one example, human spliceosomal U snRNAs are matured at their 3'-end by an exonuclease termed TOE1. This process is important because mutations in TOE1 gene can cause a human genetic disease, pontocerebellar hypoplasia (PCH). Nevertheless, TOE1 may not be the only maturation exonuclease for U snRNAs in the cell. Here, we biochemically identify two exonucleolytic factors, Interferon-stimulated gene 20-kDa protein (ISG20) and the nuclear exosome as such candidates, using a newly developed in vitro system that recapitulates 3'-end maturation of U1 snRNA. However, extensive 3'-end sequencing of endogenous U1 snRNA of the knockdown (KD) cells revealed that these factors are not the maturation factors per se. Instead, the nascent transcripts of the spliceosomal U snRNAs as well as of unstable U1 variants were found to increase in quantity upon KD of the factors. These results indicated that ISG20 and the nuclear exosome promote the degradation of nascent spliceosomal U snRNAs and U1 variants, and therefore implied their role in the quality control of newly synthesized U snRNAs.

The RNA transport factor PHAX is required for proper histone H2AX expression and DNA damage response.

PHAX (phosphorylated adaptor for RNA export) promotes nuclear export of short transcripts of RNA polymerase II such as spliceosomal U snRNA precursors, as well as intranuclear transport of small nucleolar RNAs (snoRNAs). However, it remains unknown whether PHAX has other critical functions. Here we show that PHAX is required for efficient DNA damage response (DDR) via regulation of phosphorylated histone variant H2AX (γH2AX), a key factor for DDR. Knockdown of PHAX led to a significant reduction of H2AX mRNA levels, through inhibition of both transcription of the H2AX gene and nuclear export of H2AX mRNA, one of the shortest mRNAs in the cell. As a result, PHAX-knockdown cells become more sensitive to DNA damage due to a shortage of γH2AX. These results reveal a novel function of PHAX, which secures efficient DDR and hence genome stability.

ARS2 Regulates Nuclear Paraspeckle Formation through 3'-End Processing and Stability of NEAT1 Long Noncoding RNA.

Nuclear paraspeckle assembly transcript 1 (NEAT1) is a long noncoding RNA that functions as an essential framework of subnuclear paraspeckle bodies. Of the two isoforms (NEAT1_1 and NEAT1_2) produced by alternative 3'-end RNA processing, the longer isoform, NEAT1_2, plays a crucial role in paraspeckle formation. Here, we demonstrate that the 3'-end processing and stability of NEAT1 RNAs are regulated by arsenic resistance protein 2 (ARS2), a factor interacting with the cap-binding complex (CBC) that binds to the m7G cap structure of RNA polymerase II transcripts. The knockdown of ARS2 inhibited the association between NEAT1 and mammalian cleavage factor I (CFIm), which produces the shorter isoform, NEAT1_1. Furthermore, the knockdown of ARS2 led to the preferential stabilization of NEAT1_2. As a result, NEAT1_2 RNA levels were markedly elevated in ARS2 knockdown cells, leading to an increase in the number of paraspeckles. These results reveal a suppressive role for ARS2 in NEAT1_2 expression and the subsequent formation of paraspeckles.

Translational Control of Sox9 RNA by mTORC1 Contributes to Skeletogenesis.

The mechanistic/mammalian target of rapamycin complex 1 (mTORC1) regulates cellular function in various cell types. Although the role of mTORC1 in skeletogenesis has been investigated previously, here we show a critical role of mTORC1/4E-BPs/SOX9 axis in regulating skeletogenesis through its expression in undifferentiated mesenchymal cells. Inactivation of Raptor, a component of mTORC1, in limb buds before mesenchymal condensations resulted in a marked loss of both cartilage and bone. Mechanistically, we demonstrated that mTORC1 selectively controls the RNA translation of Sox9, which harbors a 5' terminal oligopyrimidine tract motif, via inhibition of the 4E-BPs. Indeed, introduction of Sox9 or a knockdown of 4E-BP1/2 in undifferentiated mesenchymal cells markedly rescued the deficiency of the condensation observed in Raptor-deficient mice. Furthermore, introduction of the Sox9 transgene rescued phenotypes of deficient skeletal growth in Raptor-deficient mice. These findings highlight a critical role of mTORC1 in mammalian skeletogenesis, at least in part, through translational control of Sox9 RNA.

Ribosomal protein uS7/Rps5 serine-223 in protein kinase-mediated phosphorylation and ribosomal small subunit maturation.

Cellular translation should be precisely controlled in response to extracellular cues. However, knowledge is limited concerning signal transduction-regulated translation. In the present study, phosphorylation was identified in the 40S small subunit ribosomal protein uS7 (Yjr123w/previously called as Rps5) by Ypk1 and Pkc1, AGC family protein kinases in yeast Saccharomyces cerevisiae. Serine residue 223 (Ser223) of uS7 in the conserved C-terminal region was crucial for this phosphorylation event. S223A mutant uS7 caused severe reduction of small ribosomal subunit production, likely due to compromised interaction with Rio2, resulting in both reduced translation and reduced cellular proliferation. Contrary to optimal culture conditions, heat stressed S223A mutant cells exhibited increased heat resistance and induced heat shock proteins. Taken together, an intracellular signal transduction pathway involving Ypk1/Pkc1 seemed to play an important role in ribosome biogenesis and subsequent cellular translation, utilizing uS7 as a substrate.

Transport Granules Bound with Nuclear Cap Binding Protein and Exon Junction Complex Are Associated with Microtubules and Spatially Separated from eIF4E Granules and P Bodies in Human Neuronal Processes.

RNA transport and regulated local translation play critically important roles in spatially restricting gene expression in neurons. Heterogeneous population of RNA granules serve as motile units to translocate, store, translate, and degrade mRNAs in the dendrites contain cis-elements and trans-acting factors such as RNA-binding proteins and microRNAs to convey stimulus-, transcript-specific local translation. Here we report a class of mRNA granules in human neuronal processes that are enriched in the nuclear cap-binding protein complex (CBC) and exon junction complex (EJC) core components, Y14 and eIF4AIII. These granules are physically associated with stabilized microtubules and are spatially segregated from eIF4E-enriched granules and P-bodies. The existence of mRNAs retaining both nuclear cap binding protein and EJC in the distal sites of neuronal processes suggests that some localized mRNAs have not yet undergone the "very first translation," which contribute to the spatio-temporal regulation of gene expression.

Dendritic transport element of human arc mRNA confers RNA degradation activity in a translation-dependent manner.

Localization of mRNA in neuronal cells is a critical process for spatiotemporal regulation of gene expression. Cytoplasmic localization of mRNA is often conferred by transport elements in 3' untranslated region (UTR). Activity-regulated cytoskeleton-associated protein (arc) mRNA is one of the localizing mRNAs in neuronal cells, and its localization is mediated by dendritic targeting element (DTE). As arc mRNA has introns in its 3' UTR, it was thought that arc mRNA is a natural target of nonsense-mediated mRNA decay (NMD). Here, we show that DTE in human arc 3' UTR has destabilizing activity of RNA independent of NMD pathway. DTE alone was able to cause instability of the reporter mRNA and this degradation was dependent on translation. Our results indicate that DTE has dual activity in mRNA transport and degradation, which suggests the novel spatiotemporal regulation mechanism of activity-dependent degradation of the mRNA.

Exportin-5 mediates nuclear export of SRP RNA in vertebrates.

The signal recognition particle is a ribonucleoprotein complex that is essential for the translocation of nascent proteins into the endoplasmic reticulum. It has been shown that the RNA component (SRP RNA) is exported from the nucleus by CRM1 in the budding yeast. However, how SRP RNA is exported in higher species has been elusive. Here, we show that SRP RNA does not use the CRM1 pathway in Xenopus oocytes. Instead, SRP RNA uses the same export pathway as pre-miRNA and tRNA as showed by cross-competition experiments. Consistently, the recombinant Exportin-5 protein specifically stimulated export of SRP RNA as well as of pre-miRNA and tRNA, whereas an antibody raised against Exportin-5 specifically inhibited export of the same RNA species. Moreover, biotinylated SRP RNA can pull down Exportin-5 but not CRM1 from HeLa cell nuclear extracts in a RanGTP-dependent manner. These results, taken together, strongly suggest that the principal export receptor for SRP RNA in vertebrates is Exportin-5 unlike in the budding yeast.

Identification of the specific interactors of the human lariat RNA debranching enzyme 1 protein.

In eukaryotes, pre-mRNA splicing is an essential step for gene expression. We have been analyzing post-splicing intron turnover steps in higher eukaryotes. Here, we report protein interaction between human Debranching enzyme 1 (hDbr1) and several factors found in the Intron Large (IL) complex, which is an intermediate complex of the intron degradation pathway. The hDbr1 protein specifically interacts with xeroderma pigmentosum, complementeation group A (XPA)-binding protein 2 (Xab2). We also attempted to identify specific interactors of hDbr1. Co-immunoprecipitation experiments followed by mass spectrometry analysis identified a novel protein as one of the specific interactors of hDbr1. This protein is well conserved among many species and shows the highest similarity to yeast Drn1, so it is designated as human Dbr1 associated ribonuclease 1 (hDrn1). hDrn1 directly interacts with hDbr1 through protein-protein interaction. Furthermore, hDrn1 shuttles between the nucleus and the cytoplasm, as hDbr1 protein does. These findings suggest that hDrn1 has roles in both the nucleus and the cytoplasm, which are highly likely to involve hDbr1.

Crt10 directs the cullin-E3 ligase Rtt101 to nonfunctional 25S rRNA decay.

Nonfunctional mutant ribosomal RNAs in 40S or 60S subunits are selectively degraded in eukaryotic cells (nonfunctional rRNA decay, NRD). We previously reported that NRD of 25S rRNA required cullin-E3 ligase Rtt101 and its associating factor Mms1, both of which are involved in DNA repair. Although Mms22, an accessory component of the E3 complex, was suggested to direct the E3 complex to DNA repair, the factor that directs the complex to 25S NRD currently remains unknown. We herein demonstrated that another accessory component, Crt10 was required for 25S NRD, but not for DNA repair, suggesting that this accessory component specifies the function of the E3 complex differently. We also identified two distinct Crt10-containing E3 complexes, one of which contained the Paf1 complex, a Pol-II binding complex that modulates the transcription of stress-related genes. Our results showed the convergence of multiple pathways for stresses that harm nucleic acids and provided a molecular framework for the substrate diversity of the E3 complex.

Analysis of RNA transport in Xenopus oocytes and mammalian cells.

In eukaryotes, many RNA species are transcribed, processed in the nucleus, and exported to the cytoplasm, where they are destined to function or to be further matured. Some RNAs are even reimported to the nucleus. In addition, many RNAs are localized at specific nuclear bodies before their export and/or after their nuclear reimport. To understand how RNAs are transported, Xenopus oocytes are extremely useful cells, thanks to their large size. RNA transport can be easily examined by microinjecting radioactively or fluorescently labeled RNAs into Xenopus oocytes. Mammalian cultured cells are sometimes useful by virtue of RNA-FISH technique. Here, we describe methods to analyze RNA localization and export using these cells.

HIV-1 Rev protein specifies the viral RNA export pathway by suppressing TAP/NXF1 recruitment.

Nuclear RNA export pathways in eukaryotes are often linked to the fate of a given RNA. Therefore, the choice of export pathway should be well-controlled to avoid an unfavorable effect on gene expression. Although some RNAs could be exported by more than one pathway, little is known about how the choice is regulated. This issue is highlighted when the human immunodeficiency virus type 1 (HIV-1) Rev protein induces the export of singly spliced and unspliced HIV-1 transcripts. How these RNAs are exported is not well understood because such transcripts should have the possibility of utilizing CRM1-dependent export via Rev or cellular TAP/NXF1-dependent export via the transcription/export (TREX) complex, or both. Here we found that Rev suppressed TAP/NXF1-dependent export of model RNA substrates that recapitulated viral transcripts. In this effect, Rev interacted with the cap-binding complex and inhibited the recruitment of the TREX complex. Thus, Rev controls the identity of the factor occupying the cap-proximal region that determines the RNA export pathway. This ribonucleoprotein remodeling activity of Rev may favor viral gene expression.

p54nrb/NonO and PSF promote U snRNA nuclear export by accelerating its export complex assembly.

The assembly of spliceosomal U snRNPs in metazoans requires nuclear export of U snRNA precursors. Four factors, nuclear cap-binding complex (CBC), phosphorylated adaptor for RNA export (PHAX), the export receptor CRM1 and RanGTP, gather at the m(7)G-cap-proximal region and form the U snRNA export complex. Here we show that the multifunctional RNA-binding proteins p54nrb/NonO and PSF are U snRNA export stimulatory factors. These proteins, likely as a heterodimer, accelerate the recruitment of PHAX, and subsequently CRM1 and Ran onto the RNA substrates in vitro, which mediates efficient U snRNA export in vivo. Our results reveal a new layer of regulation for U snRNA export and, hence, spliceosomal U snRNP biogenesis.

Identification of a novel component C2ORF3 in the lariat-intron complex: lack of C2ORF3 interferes with pre-mRNA splicing via intron turnover pathway.

To identify the novel factors involved in the postsplicing intron turnover pathway, we carried out immunoprecipitation with known postsplicing factors, hPrp43 and TFIP11. As an interacting factor, we identified C2ORF3 protein by mass spectrometry. We found that C2ORF3 protein is present in the previously characterized Intron Large (IL) complex with an excised lariat intron. In vitro splicing using C2ORF3-depleted nuclear extracts showed significant repression of splicing, suggesting that C2ORF3 protein is required for pre-mRNA splicing through its presumable role in efficient intron turnover. Interestingly, C2ORF3 protein is localized in both the nucleoplasm and nucleoli, which suggests a potential function in rRNA processing.

hDbr1 is a nucleocytoplasmic shuttling protein with a protein phosphatase-like motif essential for debranching activity.

In higher eukaryotes most genes contain multiple introns. Introns are excised from pre-mRNAs by splicing and eventually degraded in the nucleus. It is likely that rapid intron turnover in the nucleus is important in higher eukaryotes, but this pathway is poorly understood. In order to gain insights into this pathway, we analyzed the human lariat RNA debranching enzyme1 (hDbr1) protein that catalyzes debranching of lariat-intron RNAs. Transfection experiments demonstrate that hDbr1 is localized in a nucleoplasm of HeLa cells through a bipartite type nuclear localization signal near carboxyl-terminus. The conserved GNHE motif, originally identified in protein phosphatase protein family, is critical for hDbr1 to dissolve lariat structure in vitro. Furthermore, heterokaryon experiments show that hDbr1 is a nucleocytoplasmic shuttling protein, suggesting novel role(s) of hDbr1 in the cytoplasm.

Random mutagenesis of yeast 25S rRNA identify bases critical for 60S subunit structural integrity and function.

In yeast Saccharomyces cerevisiae, 25S rRNA makes up the major mass and shape of the 60S ribosomal subunit. During translation initiation, the 60S subunit joins the 40S initiation complex, producing the 80S initiation complex. During elongation, the 60S subunit binds the CCA-ends of aminoacyl- and peptidyl-tRNAs at the A-loop and P-loop, respectively, transferring the peptide onto the α-amino group of the aminoacyl-tRNA. To study the role of 25S rRNA in translation in vivo, we randomly mutated 25S rRNA and isolated and characterized seven point mutations that affected yeast cell growth and polysome profiles. Four of these mutations, G651A, A1435U, A1446G and A1587G, change a base involved in base triples crucial for structural integrity. Three other mutations change bases near the ribosomal surface: C2879U and U2408C alter the A-loop and P-loop, respectively, and G1735A maps near a Eukarya-specific bridge to the 40S subunit. By polysome profiling in mmslΔ mutants defective in nonfunctional 25S rRNA decay, we show that some of these mutations are defective in both the initiation and elongation phases of translation. Of the mutants characterized, C2879U displays the strongest defect in translation initiation. The ribosome transit-time assay directly shows that this mutation is also defective in peptide elongation/termination. Thus, our genetic analysis not only identifies bases critical for structural integrity of the 60S subunit, but also suggests a role for bases near the peptidyl transferase center in translation initiation.

Size matters in RNA export.

In eukaryotic cells, many RNA species are exported from the nucleus to the cytoplasms. Different RNA species form distinct ribonucleoprotein (RNP) complexes for export, indicating specific RNA recognition by export proteins. Specific RNA recognition is usually achieved by specific RNA sequences or structures, but we have recently reported a molecular mechanism by which the formation of export RNP complexes is specified by RNA length. ( 1) RNA polymerase II (Pol II) synthesizes not only mRNAs but also shorter RNAs, including spliceosomal U snRNAs. Although the key U snRNA export factor, PHAX, can bind to mRNA in vitro, PHAX is excluded from mRNA in vivo. The heterotetramer of the heterogeneous nuclear RNP (hnRNP) C1/C2 specifically binds Pol II transcripts longer than 200-300 nt, and funnels them into the mRNA export pathway by inhibiting their binding by PHAX, whereas shorter transcripts not bound by the heterotetramer are committed to the U snRNA export pathway. Although this finding reveals a novel function of the C1/C2 heterotetramer and highlights the biological importance of RNA recognition by length, it has raised a number of new questions, some of which will be discussed in this article, together with some historical background of this finding.